The present invention relates to a monitoring device for a battery, a lithium-ion battery, and a method for the monitoring of a battery.
In electrically-powered vehicles, an accumulator or a battery is increasingly used as an energy source, either of which is comprised of a number of battery cells. Battery cells using lithium chemistry are often used for this purpose on the grounds that, in comparison with nickel- or lead-based batteries, the latter deliver the highest energy density available to date, with the lowest weight. In order to achieve the requisite power and energy ratings, a number of battery cells are typically connected in series. Increasingly, in response to the dictates of energy capacity or requirements for the delivery of higher power, battery cells are also connected in parallel. In total, the open-circuit voltage of the entire system is of the order of several hundred volts. Where battery cells are connected in series, a short-circuit on the vehicle-side battery pole but also, in the case of specific cell arrangements, a short-circuit on elements of the battery may lead to the flow of very high currents of up to several thousand amperes. Given that 60 volts is classified as the safe voltage for human beings, short-circuits in series circuits may result in the endangerment of persons, in case of accidents.
The failure of one cell in a parallel-connected cell composite may result in the endangerment especially of the remaining unaffected cells, and may cause the premature aging of the battery. In case of the failure of one cell in a parallel-connected cell composite, current flows in the remaining cells, resulting in the heat-up of the latter. In this case, the voltage on the remaining cells may show a normal voltage value, as a result of which the failure cannot be detected in the manner which would apply to a series circuit.
In a parallel circuit of battery cells, according to the prior art, a coupler or center tap is provided between two or more of the cells, which measures a differential voltage between the cells. Provided that the differential voltage is zero, it can be assumed that the cells are free of any operating defect. This proceeds from the fact that, in case of a fault, the voltage will no longer be evenly distributed between the cells, such that the differential voltage upon the occurrence e.g. of a short-circuit, or in case of an increase in internal resistance associated with the failure of a cell, or in case of any other faults, will no longer be zero. This can be measured and, on the basis of the magnitude and the polarity of the differential voltage, the nature of the fault and the defective part of the battery can be identified.
This fault identification is known, for example, from DE 3438783 C1. DE 3438783 C1 discloses a monitoring device for a battery with at least two parallel-connected branches, each of which comprises cell rows of m+n cells. This monitoring device can detect faults in the operation of the battery and the failure of individual battery cells, whereby a battery of symmetrical structure is required for this purpose. To this end, in at least two of the parallel-connected cell rows of the battery, between at least the mth and the (m+1)th cell respectively, a center tap is connected for an electrical measurement line, which is connected to a circuit arrangement for the measurement of a differential voltage. According to the disclosure of DE 3438783 C1, the differential voltage permits conclusions to be drawn with regard to the condition of the battery elements, and consequently of the cell rows. Provided that both cell rows and their constituent individual cells are operating in a fault-free manner, the differential voltage in all steady-state operating conditions will be equal to zero. Upon the occurrence of a short-circuit in a cell, an increase in the internal resistance of a cell, a contact fault between two cells, and the overloading or exhaustive discharge of a cell, the differential voltage will deviate from zero. From the magnitude and polarity of the differential voltage, and from the relevant operating state of the battery, it is possible to identify the nature of the fault and the battery element in which the fault has occurred.
Printed publication WO 90/10334 A1 also discloses a circuit for the monitoring of a rechargeable battery with parallel-connected battery cells. To this end, the circuit is configured for the monitoring of voltage differentials between the parallel branches and, in the event of a deviation which exceeds a predetermined threshold, for the generation of a fault signal. According to the disclosure of WO 90/10334 A1, a defective battery is also affected by heat-up, such that a thermal release may be provided for protection against overheating.
From printed publications DE 102012201528 A1 and DE 102010062856 A1, it is also known that the temperature of battery cells is determined by means of temperature sensors, whereby it is also proposed that a check should be executed in order to confirm that the recorded operating parameters lie within permissible ranges.
More specifically, DE 102012201528 A discloses a method for the monitoring of the temperature of a temperature-sensitive system, specifically a battery pack with series- and/or parallel-connected storage cells. To this end, for the recording of the temperature of battery cells, temperature sensors or temperature-responsive resistors are arranged on the battery cells.
DE 102010062856 A1 discloses a method for the determination of the operating parameters of a battery, a battery management system, and a battery. The battery management system is provided with means for the measurement of operating parameters, for example the battery voltage, battery current and battery temperature. Permissible operating ranges for various battery state variables, including temperature, state of charge, aging and current intensity, each with reference to the other operating parameters, are stored in a memory. A device is also provided for the predetermination of the requisite operating properties of the battery. In summary, DE 102010062856 A1 proposes a system or a calculation method whereby the monitoring of appropriate parameters, including temperature, state of charge or current intensity, is performed, such that the working point does not lie outside the permissible operating range, thereby resulting in the more rapid aging of the battery and, accordingly, a shorter service life of the battery.